The kinetics and thermodynamics of clay mineral reactions

The diagenesis of rocks during burial occurs in response to changing temperature, pressure, and solution composition. Due to their geologic abundance, high surface area, and reactivity clay minerals are important participants in the diagenesis of clastic rocks. The kinetic and thermodynamic stability of clays is in general poorly understood. This dissertation research measured the rate of transformation of kaolinite to muscovite/illite and developed a method to estimate clay mineral thermodynamic stability.

Clastic rock diagenesis is controlled by the rates of silicate mineral growth and transformation. Marine mudstones commonly contain large proportions of kaolinite which reacts during diagenesis to form muscovite/illite and/or chlorite. Batch reactor experiments were used to measure the reaction rate of 1.5 kaolinite + K⁺ = muscovite + H⁺ + 1.5 H₂O using the initial rate method and a fitted form of the integrated rate equation. Experiments were performed at temperatures ranging from 250° to 307°C with solutions of 0.5 - 2.0 m KCl. These results can then be extrapolated to diagenetic temperatures using the Arrhenius equation.

ln addition, a technique was developed to estimate the ΔG_f0 and ∆H_f0 of silicate minerals. Silicate minerals have been shown to act as a combination of basic polyhedral units (Hazen 1985 and 1988). This work showed that their thermodynamic properties could be modeled as the sum of polyhedral contributions. A multiple linear regression model was used to find the contribution of the oxide and hydroxide components (gᵢ and hᵢ) to the ΔG_f0 and ∆H_f0 of a selected group of aluminosilicate minerals at 298 K. The ΔG_f0 and ∆H_f0 of other silicate minerals can be estimated from a weighted sum of the contribution of each oxide and hydroxide component (gᵢ and hᵢ). These results can be also used to estimate the ΔG_f0 of silicate minerals at higher temperatures (up to =600 K) by using the equation,

gᵢ(T)= hᵢ(298) - T((hᵢ(298)-gᵢ(298))/298)